How can I fix:?

Vokera Linea 24 combi boiler.
Boiler starts up fine. Water heating fine, although preasure does gradually drop over time.
However when the central heating is turned on, the preasure goes very high. Radiators heat up very quickly..which is great.But then water drips out of emergancy release valve. Preasure drops quickly and system has to be turned off and re-preasurised.
Radiators have been bled, and no leeks visible.